Amalgam seal for alkali metal chloride electrolytic cells



Nov. 21, 1967 w, HONSBERG ET AL 3,354,071

AMALGAM SEAL FOR ALKALI METAL CHLORIDE ELECTROLYTIC CELLS File Feb. 1964 IIII'IIIIIII INVENTORS. WERNER HONSBERG BY HEINRICH MUELLER WZMQ p612;

ATT'YS United States Patent 3 Claims. oi. 204-419 This invention relates to an alkali metal chloride electrolytic cell and particularly to an amalgam seal arranged at the end of such a cell.

Alkali metal chloride electrolytic cells operating according to the mercury method usually comprise a wide trough whose bottom is covered by flowing mercury. The mercury is cathodically polarized during operation and alkali metal is separated thereon electrolytically. During its passage through the electrolytic cell, the mercury takes up from 0.1 to 0.8% by weight of alkali metal depending on the current loading and the throughput of mercury. The alkali metal amalgam formed leaves the electrolytic cell by passing through an amalgam seal and it then passes direct or after having flowed through a separate washing chamber into the denuder. There the amalgam is decomposed with water in the presence of graphite granules so that alkali metal hydroxide solution, hydrogen and almost pure mercury are formed. The latter is pumped back into the electrolytic cell.

A form of amalgam seal which is often used in industry is shown diagrammatically in section in FIGURE 1 of the accompanying drawing. Amalgam 1 flows from the chamber 2 of an electrolytic cell through a weir 4 into a denuder chamber 3. To prevent build-up in the electrolytic cell it is necessary that there should be a diflerence between the level of the amalgam in the weir on the cell side and the level of the amalgam in the cell itself. The thickness of the layer of amalgam as it passes from the cell into the weir is less, as shown at 5 in FIGURE 1, than the thickness of the layer of amalgam in the cell, because of the higher rate of flow. The level of the amalgam on the cell side of the weir is determined by the salt solution present in the cell and the gas pressure of the chlorine. The level of the amalgam on the cell side of the weir should never be allowed to become higher than the level of the bottom 6 of the cell. The partition 7 should dip into the amalgam for a distance of about 20 to 30 mm.

A seal of this type has disadvantages. It is impossible to prevent salt solution from being entrained from the electrolytic cell into the denuder with the flowing amalgam in which the bulk of the entrained salt solution is present in dispersed form. To obviate this disadvantage, the outlet 8 from the weir is chosen farily wide in order to afford the amalgam in the weir the opportunity to calm and allow the enclosed salt solution to rise into the cell. In spite of this measure, the separation of the salt solution is incomplete so that in large mercury cells having a loading of 50 to 200' kiloamp., the about 50% caustic alkali liquor leaving the denuder contains 50 to 150 mg./l. of NaCl if water free from chloride has been used for the decomposition.

Entrainment of the salt solution may also be curbed by providing a separate compartment between the cell and the denuder in which the amalgam flowing through is washed with water.

Both methods involve an increased consumption of mercury. Furthermore a viscous mass for-ms in the washing compartment so that operation of the electrolytic cell and working up of the amalgam are made much more d iflicult. This mass (known as mercury butter) usually consists of amalgam having liquids and gases dispersed in it.

We have now found that by providing a trough filled with amalgam into which a partition partly projects downwardly and by providing obstacles in the path of the amalgain from the outlet from the electrolytic cell to the trough it is possible to prevent practically completely the entrainment of salt solution with the flowing mercury cathode from the electrolytic cell into the amalgam denuder, when the obstacles are arranged on a surface which is offset from and lower than the bottom of the electrolytic cell and communicates with the trough and when the obstacles are arranged over the whole width of the surface and transversely to the direction of flow of the amalgam.

A convenient embodiment of seal according to this invention is illustrated diagrammatically in FIGURE 2 of the accompanying drawing. Amalgam 10 flows from the cell chamber 11 through the weir 15 into the decomposition chamber 12. Prior to entry into the weir 15, the amalgam flows over a horizontal or, in the direction of the mercury flow, slightly inclined surface 16 which is provided with obstacles 17 in the form of ribs arranged transversely to the direction of flow of the amalgam. The surface 16 is offset downwardly with respect to the bot tom 14 of the cell and communicates with the trough of the weir 15. The level of amalgam on the cell side of the outlet weir is advantageously kept at such a height that the obstacles 17 remain covered by about 5 mm. of amalgam. This is easily achieved by keeping the level of the amalgam at the outlet side of the weir at a greater or less height according to the columns of liquid in the cell and in the decomposer and according to the gas pressures on both sides of the weir. By the diiference in level of the amalgam on the cell side of the weir and in the cell itself, the thickness of the layer of amalgam is decreased by the increased rate of flow, as indicated at 13 in FIGURE 2, and accumulation in the cell is avoided.

An amalgam outlet weir of the type according to this invention has a very effective arresting action for entrained salt solution. When operating large mercury cells having a loading of about 50 to 200 kiloamp., the 50% alkaline liquor produced in the denuder exhibits a common salt content of 3 to 12 mg. per liter if water which is free from chloride is used for the decomposition. A further advantage of the weir according to this invention is that the passage 18 of the weir may be kept very narrow. The width of the passage 18 is usually only 10 to 20 mm. as compared with 30 to 50 mm. for the passage 8 of the prior art weir illustrated in FIGURE 1, so that considerable amounts of mercury are saved in the filling of the cell.

We claim:

1. A horizontally flowing mercury cathode cell having a mercury amalgam seal at the outlet end thereof, said mercury seal comprising: a transverse trough extending below the bottom of said cell and adapted to contain amalgam into which a transverse partition projects downwardly, said bottom of said cell having a downwardly oflset portion thereof adjacent said trough and on the inlet side of said trough, and a plurality of transverse projections extending upwardly from the bottom of said 4 References Cited UNITED STATES PATENTS 1/1943 Gardiner 204-219 8/ 1958 Neipert et a1 204219 JOHN H. MACK, Primary Examiner.

D. R. VALENTINE, Assistant Examiner. 

1. A HORZONTALLY FLOWING MERCURY CATHODE CELL HAVING A MERCURY AMALGAM SEAL AT THE OUTLET END THEREOF, SAID MERCURY SEAL COMPRISING: A TRANSVERSE TROUGH EXTENDING BELOW THE BOTTOM OF SAID CELL AND ADAPTED TO CONTAIN AMALGAM INTO WHICH A TRANSVERSE PARTITON PROJECTS DOWNWARDLY, SAID BOTTOM OF SAID CELL HAVING A DOWNWARDLY OFFSET PORTION THEREOF ADJACENT SAID TROUGH AND ON THE INLET SIDE OF SAID TROUGH, AND A PLURALITY OF TRANSVERSE PROJECTIONS EXTENDING UPWARDLY FROM THE BOTTOM OF SAID OFFSET PORTION, SAID PROJECTIONS BEING ADAPTED TO PROVIDE OBSTACLES IN THE PATH OF THE FLOWING AMALGAM AS IT ENTERS THE INLET SIDE OF SAID TROUGH. 